Large mode area fibers using higher order modes

ABSTRACT

The specification describes an optical fiber device wherein a LOM is converted to an HOM prior to entering the gain section. The gain section is a few mode fiber that supports the HOM. The output from the gain section, i.e. the HOM, may be utilized as is, or converted back to the LOM. With suitable design of the few mode fiber in the gain section of the device, the effective area, Aeff, may be greater than 1600 μm 2 . The large mode separation in the gain section reduces mode coupling, allowing greater design freedom and reducing the bend sensitivity of the optical fiber.

RELATED APPLICATION

This application is a Continuation of application Ser. No. 10/990,088,filed Nov. 16, 2004.

FIELD OF THE INVENTION

This invention relates to optical fiber devices, such as lasers andamplifiers, that produce high power levels through the use of largeeffective mode area. More specifically, the devices derive improvedperformance characteristics when the gain element is deliberatelyoperated in a higher order mode.

BACKGROUND OF THE INVENTION

(Parts of the following section may not be prior art.)

Fiber lasers with high pulse energy, good beam quality and excellentoptical characteristics have applications in many fields and industriessuch as materials processing (marking, welding, semiconductor wafer andmask repair etc), medical and industrial spectroscopy (fluorescence,absorption), illumination, remote sensing and spectroscopy (wind speed,biohazards, ecosystem mapping etc), ranging and targeting (collisionavoidance, military applications etc) and scientific instrumentation.For reasons of simplicity and efficiency, Yb³⁺-doped fibers are mostcommonly used. They can be optically pumped from 915 nm-975 nm andachieve emission from 975-1100 nm with optical conversion efficiency ashigh as 70%. Currently, advances in this field are primarily constrainedby limitations in maximum extractable energy, and the onset of nonlinearimpairments. Saturation energy of the gain medium is a key parameter fordetermining how much energy can be stored in an amplifier, and is givenby^(i) $\begin{matrix}{E_{sat} = \frac{{h\nu}_{s}A_{eff}}{\left( {\sigma_{es} + \sigma_{as}} \right)\Gamma_{s}}} & (1)\end{matrix}$where σ_(es), σ_(as) are the emission and absorption cross section atthe signal wavelength, hν_(s) is signal energy at frequency ν_(s),A_(eff) is area of the active doped region and Γ_(s) is signal overlapwith the active dopant. As a general rule, the extractable energy storedin a fiber is limited to around ten times the saturation energy. As anexample, for standard single mode Yb³⁺ doped fiber with 8□m corediameter, E_(sat)=0.04 mJ, indicating extraction of only about 0.4 mJper pulse.

Two deleterious nonlinear effects of concern are stimulated Brillouinscattering (SBS) and stimulated Raman scattering (SRS). Both rob powerfrom the signal and can cause catastrophic damage. For SRS, thethreshold for peak power P_(th) before onset of serious Raman scatteringin passive fibers is given by: $\begin{matrix}{P_{th} = \frac{16A_{eff}}{g_{R}L}} & (2)\end{matrix}$where A_(eff) is the effective mode area of the fiber, g_(R) is theRaman gain coefficient and L is the fiber length. For a fiber with 25 μmcore diameter, P_(th)·L˜70 kWm. Since typical fiber lengths exceed 5meters, this indicates peak powers of only 20 kW before Raman scatteringbecomes severe.

Stimulated Brillouin scattering arises from interaction of the signalwith longitudinal acoustic modes of the fiber, causing part of thesignal to be reflected backwards. Similar to the case of SRS, thethreshold condition for SBS can be written as: $\begin{matrix}{P_{th} = {\frac{21A_{eff}}{g_{B}L}\left( {1 + \frac{BW}{{BW}_{{SiO}_{2}}}} \right)}} & (3)\end{matrix}$where g_(B) is the Brillouin gain coefficient, BW is the bandwidth ofthe signal and BW_(SiO2) is the Brillouin bandwidth of a silica, i.e.SiO₂, fiber (˜50 MHz for silica). If the signal has bandwidth comparableto BW_(SiO2), then for a fiber with 25 μm core diameter, P_(th)·L˜350Wm. This is obviously a severe constraint and mitigation is desirable.For both SBS and SRS impairments, equations (3) and (4) indicatemitigation is possible by increasing the modal area and decreasing thefiber length. Because a larger core occupies a larger fraction of theoverall fiber cross-section and therefore has higher pump absorption,the optimum fiber length varies inversely with A_(eff). Thus, increasingthe core area naturally results in shorter length. Since the nonlineareffects vary as A/L, the increase in threshold varies as A_(eff) ².

Currently, the practical solution for obtaining large A_(eff) fiber isconceptually straightforward—simply increasing the core diameter. Thisresults in monotonically increasing A_(eff) of the signal. However,there are several limitations to this approach. For single-modeoperation, as the core diameter increases, the refractive indexdifference between the core and cladding, Δn, must decrease. IfΔn<0.001, though, the fiber becomes bend sensitive. And when Δn is fixedat a minimum, further increase in core diameter results in multimodeoperation. While this is permissible, core size is then constrained byunavoidable but undesirable energy transfer among modes.

The mode coupling efficiency η between modes in a multimode fiber isgiven by $\begin{matrix}{\eta \sim \frac{\lambda^{2}\kappa^{2}}{\Delta\quad n_{eff}^{2p}}} & (4)\end{matrix}$where κ is the perturbation amplitude due to index and microbendfluctuations, Δn_(eff) is the difference in effective indices betweendifferent modes, and p is a fitting parameter (with value >0) to accountfor mechanical perturbations on a fiber. Thus, large Δn_(eff) (e.g.>8×10⁻⁵) is desirable for low mode coupling. Unfortunately, as A_(eff)increases, Δn_(eff) decreases and rapidly asymptotes to values muchsmaller than 8×10⁻⁴, and mode coupling cannot be reduced. This isillustrated in FIG. 1, which shows simulations of two designs forachieving A_(eff) ˜1600 μm² (mode field ˜45 μm). FIG. 1 a shows therefractive index profiles of the designs considered. The fiber withhigher Δn has Δn_(eff)=6×10⁻⁵, indicating that it is highly susceptibleto mode coupling. Note that this mode has negligible bend loss, as shownin FIG. 1 b. Even with a huge reduction in Δn, Δn_(eff) is onlyincreased by 30% and mode coupling remains catastrophic. Note that thisreduction in Δn leads to extreme bend loss (FIG. 1 b).

FIG. 1 c illustrates an additional problem with large A_(eff) designs.All applications of high power lasers and amplifiers involve spatiallytransforming and focusing the device output. This is best achieved withGaussian beams. Thus, an important metric for high power devices is theM² of the output light, where M² is a measure of the departure from aperfect Gaussian spatial profile (M²=1 is a perfectly Gaussian mode),given by: $\begin{matrix}{M^{2} = \frac{\int{r^{2}E^{2}{r \cdot {\mathbb{d}r}}}}{\int{\left( {{\mathbb{d}E}/{\mathbb{d}r}} \right)^{2}{r \cdot {\mathbb{d}r}}}}} & (5)\end{matrix}$where E is the electric field profile of the mode, and r is the radialcoordinate. FIG. 1 c shows two mode profiles representing two differentM² values for the two different designs (low and high mode coupling)represented in FIG. 1 a. The output beam, becomes highly distorted (M²dramatically increases) for the design with low mode coupling, and issensitive to index perturbations in the core. Very tight control offiber fabrication conditions is therefore necessary to maintain goodbeam quality, and this is difficult in fibers with A_(eff)>350 μm².

Current preferred laser designs concentrate on means to force operationin a fundamental mode, even though the fiber may guide several modes.One disclosed means to achieve this is to preferentially strip thehigher order modes (HOM). While this may be adequate for moderateA_(eff), the higher modal content of large A_(eff) fibers leaves littleroom for discrimination of bend loss between modes. Alternatively,gain-inducing dopants can be selectively deposited in a fiber preform sothat only the fundamental mode is substantially amplified or guided.While this technique would allow amplification of the desired mode incomparison to HOMs, it is designed for cases where the fundamental modeis substantially spatially separated from the HOMs—a condition typicallyabsent in very large A_(eff) fibers. Another approach is to dope thefiber in a ring around the core rather than in the core itself. Thisincreases the gain saturation limit of the gain medium, allowingextraction of higher power pulses. However, this technique leads tosignificant degradation of the output mode profile, i.e. departure fromM²=1. Since many of the HOMs overlap spatially, mode coupling and modediscrimination becomes problematic.

Given the numerous performance trade-offs, gain fibers with currenttechnology face a practical limit of mode field diameter ˜20 μm(A_(eff)=350 μm²) with little prospect of future advances usingconventional engineering expedients. Thus there exists a need for anamplifier fiber that simultaneously yields very large A_(eff), low modecoupling, and good output beam quality.

STATEMENT OF THE INVENTION

We have developed a new approach to the realization of optical fiberdevices with very large mode area, good bend loss performance, largespacing between guided modes (for low mode coupling), and good beamquality (M²˜1). These properties are produced, according to theinvention, by using a few mode optical fiber, and converting the inputsignal to a higher order mode. This yields significant designflexibility, so that all the desirable properties (large A_(eff), lowbend loss, low mode coupling and M²˜1) can be simultaneously achieved.

Two embodiments of fiber designs suitable for implementing the inventionare described below. These are illustrative of optical fibers whereinthe HOM is the LP02 mode, but the invention can be implemented with anyHOM guided by the fiber. It may also be implemented using a conversionof lower mode input (LOM) to HOM.

The first design class (called the ring design, henceforth) illustratesa fiber with a central core and one or more high index rings followed bya down-doped region. The second design class (called the truncatedcladding design, henceforth) comprises a central core and an innercladding, followed by a broad down-doped region spaced significantlyfrom the center of the fiber. Both of these designs can yield A_(eff)for the LP₀₂ mode ranging up to 2800 μm² and beyond. Furthermore, thedeep-down-doped regions ensure that the HOM is not radiated, and thusgood bend loss performance is obtained. The enhanced design flexibilityfor HOM fibers enables designing them with large effective indexseparations (Δn_(eff)˜the difference in effective index between the LP₀₂mode and any other guided mode). Designs guiding the LP₁₂ mode inaddition to the desired LP₀₂ mode yield Δn_(eff)>8×10⁻⁵, while designsthat guide only the LP₀₁, LP₁₁ and LP₀₂ modes yield Δn_(eff) as high as3×10⁻³. Thus, these fibers exhibit very low mode coupling problems. Inaddition, the LP₀₂ mode is vastly spatially separated from other guidedmodes in the fiber. Thus, preferential gain-guiding mechanisms attemptedearlier for fundamental mode gain fibers, can be readily applied here tofurther increase modal discrimination and decrease the deleteriouseffects of mode mixing. Another advantage of these fibers is that whilethe signal propagates in the LP₀₂ mode, light enters/exits the fiber inthe fundamental, LP₀₁ mode. The characteristics of this mode aregoverned by the central core, while those of the LP₀₂ mode are governedby other features. Hence, the central core can be designed to yield aLP₀₁, mode with mode profile almost indistinguishable from a perfectGaussian mode profile.

In some cases, for efficient operation, it is helpful to add suitablecore dopants that necessarily increase the core index. For example Er/Ybfibers use high phosphorous concentrations. High Δ cores avoid use oflarge MFD if fundamental mode operation is desired. HOMs can be designedto propagate in high Δ regions.

Since signal propagation is in the LP₀₂ mode rather than the fundamentalmode, the fiber is provided with mode-converters to convert the incomingsignal in the fundamental mode, into the HOM. The output signal mayeither be down-converted to the LP₀₁ mode, or focused/collimated as is.Mode converters also function as wavelength selective filters, such as abandpass filter. These are useful for filtering out unwanted ASE rstokes shifted light. Thus, use of HOMs is advantageous even if A_(eff)is not large.

The inventive features may be more easily followed with the aid of thedrawing:

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 a-1 c show performance characteristics that the invention isaimed at improving;

FIG. 2 is a schematic of a refractive index profile for a ring designfiber of the invention;

FIG. 3 is a refractive index profile as well as a mode distribution plotfor the optical fiber of FIG. 2;

FIG. 4 is a schematic of a refractive index profile for a truncatedcladding design fiber of the invention;

FIG. 5 is a refractive index profile as well as a mode distribution plotfor the optical fiber of FIG. 4;

FIG. 6 is a plot comparing the optical fibers of FIGS. 3 and 5 withrespect to effective area variation with wavelength;

FIG. 7 is a schematic diagram of the overall system of the invention;

FIG. 8 a is a schematic diagram representing a long period grating (LPG)switchable mode converter showing input and output mode distributionspectra;

FIG. 8 b is a plot showing typical spectral features of the LPGs of FIG.8 a using different mode-converting efficiencies;

FIGS. 9 a and 9 b illustrates devices with an HOM converter at the inputonly;

FIGS. 10 a-10 c illustrate pumping and mirror arrangements suitable foruse with the invention.

DETAILED DESCRIPTION

As mentioned previously, two distinct embodiments of fiber designssuitable for the gain section of the devices of the invention will bedescribed. These are illustrative of optical fibers wherein the HOM isthe LP02 mode, but the invention can be implemented with any HOM guidedby the fiber. It should also be understood that other optical fiberdesigns may be found useful for obtaining the high effective areaperformance demonstrated by the two designs shown here. These opticalfibers are examples of a category of optical fibers known as few modefibers. They have a mode field larger than a single mode fiber.

The ring design optical fiber has a central core and one or more highindex rings (with Δn>2×10⁻³) of thickness greater than 2 μm, followed bya deep down doped region (Δn<−0.003) with a thickness of at least 5 μm.The high index ring exists at a radial position greater than 20 μm fromthe center of the fiber. The desired mode field diameter is at least 20microns, preferably >40 μm. For designs utilizing the wavelengthselectivity of mode converters, this invention is advantageous even withsmall (6-10 μm) MFD fiber.

FIG. 2 shows the canonical refractive index profile for a ring designfiber, and is characterized by the values ΔN_(core), ΔN_(lclad),ΔN_(ring), ΔN_(dd), and ΔN_(oclad), representing the refractive indexvalues of the core, inner clad, ring, deep down-doped and outer cladregions, respectively. The radial positions of these index features isgoverned by d_(core), d_(iclad), d_(ring), d_(dd), and d_(oclad),representing the thicknesses of the core, inner clad, ring, deepdown-doped and outer clad regions, respectively. The central core hasrefractive index Δn_(core), and thickness d_(core), such that the LP₀₁and LP₁₁ modes substantially reside within it. Hence, their modalproperties are governed almost exclusively by this region. On the otherhand, the inner cladding, ring and deep down-doped regions govern theproperties of the LP₀₂ mode.

FIG. 3 shows a typical refractive index profile for this design class,along with the modal profiles for the LP₀₁, LP₁₁, and LP₀₂ modes. It isimmediately evident that the LP₀₂ power resides in regions substantiallyseparated from the LP₀₁ and LP₁₁ modes. Thus, preferential amplificationor gain-guiding mechanisms can be readily applied to this design, toamplify only the LP₀₂ mode in comparison to other modes. For moredetails of these mechanisms see U.S. Pat. No. 5,187,759, the content ofwhich is incorporated herein by reference. This fiber has A_(eff)˜2100μm², and only guides the LP₀₁, LP₁₁ and the desired LP₀₂ mode. Thedifference in effective indices between nearest neighbors,Δn_(eff)=3×10⁻³, which is at least an order of magnitude larger thanthat in conventional, fundamental mode fibers with substantially lowerA_(eff). This illustrates the vast large A_(eff) design space accessibleto HOMs. Furthermore, the design parameters (as given by the refractiveindex values and thickness values illustrated in FIG. 2) can be modifiedto yield fibers with vastly different A_(eff). This is illustrated inthe following Table, which shows the variation of the parameters ofinterest, as a function of d_(iclad), the thickness of the inner cladregion. TABLE d_(clad) A_(eff)(μm²) Minimum Δn_(eff) M² 19 1670 3.894 ×10⁻³ 1.02 22 2088 3.890 × 10⁻³ 1.02 24 2386 3.888 × 10⁻³ 1.02 27 28603.886 × 10⁻³ 1.02

Note that for LP₀₂ mode A_(eff) ranging from 1600 to 2800 μm², theoutput mode shape (M²˜1.02) remains the same, as expected, since theoutput mode is governed by the LP₀₁ mode and not the LP₀₂ mode. Inaddition, Δn_(eff) also remains larger than 10⁻⁴ for all these designs,indicating that they are robust with respect to mode coupling problems.

The truncated cladding design optical fibers comprise a central core, aninner cladding of index similar to that of silica, followed by a deepdown-doped region (ΔN<−0.003), at a radial position greater than 20 μmfrom the center of the fiber. The down-doped region has a thicknessgreater than 5 μm, and can extend to the periphery of the fiber.

FIG. 4 shows the canonical refractive index profile for a truncatedcladding design fiber, and is characterized by the values Δn_(core),Δn_(iclad), Δn_(dd), and Δn_(oclad), representing the refractive indexvalues of the core, inner clad, deep down-doped and outer clad regions,respectively. The radial positions of these index features is governedby d_(core), d_(iclad), d_(dd), and d_(oclad), representing thethicknesses of the core, inner clad, deep down-doped and outer cladregions, respectively. The central core has refractive index Δn_(core),and thickness d_(core), such that the LP₀₁ and LP₁₁ modes substantiallyreside within it. Hence, their modal properties are governed almostexclusively by this region. On the other hand, the inner cladding anddeep down-doped regions govern the properties of the LP₀₂ mode.

FIG. 5 shows a typical refractive index profile for this design class,along with the modal profiles for the LP₀₁, LP₁₁, LP₁₂ and LP₀₂ modes.It is immediately evident that the LP₀₂ power resides in regionssubstantially separated from the LP₀₁ and LP₁₁ modes. However, unlike inthe case of the ring designs, the LP₁₂ mode has strong spatial overlapwith the LP₀₂ mode. Thus, preferential amplification or gain-guidingmechanisms will not be expected to provide additional modaldiscrimination in this case: While this is a drawback in comparison tothe ring designs, this profile is more robust to manufacturingvariations in comparison to the ring designs. This is illustrated byFIG. 6, which shows the variation of A_(eff) with respect to operatingwavelength, for the two design classes.

The fiber illustrated in FIG. 5 fiber has A_(eff)˜2150 μm², anddifference in effective indices between nearest neighbours,Δn_(eff)=9×10⁻⁵, which implies that the mode coupling performance ofthese fibers will also be adequate. As in the case of the ring designs,the thickness of the inner clad d_(iclad), can be varied to yield fiberswith a variety of A_(eff) ranging from 1600 to 2800 μm². Again, theoutput mode shape (M²˜1.02) is as close to Gaussian, as expected, sincethe output mode is governed by the LP₀₁ mode and not the LP₀₂ mode.

In the truncated cladding designs, the bend loss for the LP₀₂ mode iscontrolled by the thickness of the deep down-doped region and the outerclad region. However, the spatial overlap between the LP₁₂ and LP₁₂ modeis also controlled by the deep down-doped region. As a general rule, thethickness of the deep down-doped region d_(dd), can be increased at theexpense of d_(oclad) (in the limiting case, d_(dd) can be made largeenough to extend throughout the fiber, while eliminating the outer clad,i.e. d_(clad)=0), to increase the confinement of the LP₀₂ mode withoutsacrificing its A_(eff). However, this parameter must be optimized withrespect to the degree of spatial overlap between the LP₁₂ and LP₀₂modes, and thus the ideal operating point in this design space is ford_(dd) ranging from 5-15 μm.

Both of these designs can be engineered to yield A_(eff) for the LP₀₂mode ranging from 1600 μm² to 2800 μm². Furthermore, the deep-down-dopedregions ensure that the HOM is not radiated, and thus good bend lossperformance is obtained. The enhanced design flexibility for HOM fibersenables designing them with large effective index separations(Δn_(eff)—the difference in effective index between the LP₀₂ mode andany other guided mode). Designs guiding the LP₁₂ mode in addition to thedesired LP₀₂ mode yield Δn_(eff)>8×10⁻⁵, while designs that guide onlythe LP₀₁, LP₁₁ and LP₀₂ modes yield Δn_(eff) as high as 3×10⁻³. Thus,these fibers exhibit very low mode coupling problems. In addition, theLP₀₂ mode is vastly spatially separated from other guided modes in thefiber so that preferential gain-guiding mechanisms attempted earlier forfundamental mode gain fibers, can be readily applied here to furtherincrease modal discrimination and decrease the deleterious effects ofmode mixing. Another advantage of these fibers is that while the signalpropagates in the LP₀₂ mode, light enters/exits the fiber in thefundamental, LP₀₁ mode. The characteristics of this mode are governed bythe central core, while those of the LP₀₂ mode are governed by otherfeatures. Hence, the central core can be designed to yield a LP₀₁ modewith mode profile metric M²˜1.02, which is almost indistinguishable froma perfect Gaussian mode profile.

Mode converters for converting the incoming, and optionally theoutgoing, signals between modes may be of any suitable design. The modeconverting functionality may be achieved within the gain fiber usingin-fiber grating mode converters. Alternatively, holographic free-spacemode converters, or tapered hollow-core fibers, may be employed. Suchmode converters can be designed to be broadband or spectrally selective,depending on whether the application is a laser or amplifier. Thisoffers the additional advantage of spectral filtering to reduce noisefrom amplified spontaneous emission (ASE). Moreover, the mode convertersare by definition mode-selective, and hence offer an additional degreeof modal discrimination, further decreasing mode-coupling problems.

The fiber designs disclosed above enable propagation of an HOM (in theillustrated examples, the LP₀₂ mode) with A_(eff) ranging from 1600 to2800 μm², and low mode coupling susceptibility. However, in addition thefibers are provided with means to access the HOM. The mode profiles forthe LP₀₂ mode, depicted in FIGS. 3 and 5 show that they have two powermaxima, and are very distinct in shape from the Gaussian profilenormally employed in a free-space or conventional fiber apparatus.Hence, the incoming signal in the examples given is converted into theLP₀₂ mode. In addition, some applications may also require that theamplified output also be converted into a Gaussian profile, and use anoutput mode converter that performs the reciprocal function. This isillustrated in FIG. 7, which shows the HOM fiber discussed aboveconnected to mode-converting couplers at the input and outputrespectively. These mode converters transform the incoming light fromthe LP₁₁ or some Gaussian mode to the LP₀₂ mode in the fiber. Thereverse—reciprocal action—is realized with the mode converter at theoutput, which yields a Gaussian output for the device. The input fiberis typically a single mode transmission fiber, or a few mode fiber thatstrongly guides the fundamental LP01 mode so that the optical signalentering the device is predominantly in the LP01 mode.

A preferred means to obtain the mode-converting device functionality iswith co-propagating long period fiber gratings (LPG). LPGs may beinduced in the HOM fiber itself, enabling a low cost, low lossmode-converting device. Such gratings may be made narrowband, if onlyone wavelength of operation is required, or can be arbitrarilybroadband. Mode converters are also known that cover a wavelength rangeas large as 500 nm. For more details see S. Ramachandran, M. Yan, E.Monberg, F. Dimarcello, P. Wisk and S. Ghalmi, “Record bandwidthmicrobend gratings for spectrally flat variable optical attenuators,”IEEE Photon. Tech. Lett., vol. 15, pp. 1561-1563, 2003; S. Ramachandran,U.S. patent application Ser. No. 10/234,289, both of which areincorporated by reference herein.

Suitably designed LPGs can be both static as well as tunable in theirmode coupling strength. This is illustrated in FIG. 8, which shows theschematic of switchable mode-conversion enabled by LPGs, along with theinput and output mode profiles (FIG. 8 a), as well as the typicalspectral features of these gratings tuned to a variety ofmode-converting efficiencies (FIG. 8 b). Since LPGs provide spectralfiltering as well as modal discrimination in addition to modeconversion, they enable HOM amplifier schematics (of the kind shown inFIG. 7, for example) to be inherently low noise.

Several other devices may be used in lieu of LPGs to achieve theLP01-LP₀₂ mode-converting functionality depicted in FIG. 7. Examples ofalternate mode converters include:

-   -   1) Elements offering spatially selective phase delays can be        used to assemble free-space couplers that offer broadband,        efficient mode conversion. Hence, the device can be used both to        up- as well as down-convert the signal from the LP₀₁ to the LP₀₂        mode, and vice versa.    -   2) Beam shaping elements of various kinds have been used to        spatially transform a beam of light. Examples include        combinations of diffractive lenses, lens arrays and combinations        to provide astigmatic corrections.

Typically, such elements are used to change the aspect ratio of aspatial pattern, as is needed to couple light from laser diodes tofibers, but the concept can be extended to change the spatial patternbetween modes of a fiber too.

All the mode converters described above can be used to offer the spatialmode transformation between the LP₀₁ and LP₀₂ modes, as depicted in theschematic of FIG. 7. An alternative schematic is also useful, as shownin FIG. 9. Here the input signal is converted to the LP₀₂ mode, usingany of the mode converting schemes defined above, but the output is nottransmitted through a mode converter. Hence, the light exiting thedevice is in the LP₀₂ mode. This may subsequently be propagated in freespace, using standard collimating lenses (FIG. 9 a), or be convertedinto any desired beam shape with the use of free-space beam transformersdescribed above (FIG. 9 b). The prospect of free-space collimation andpropagation of the LP₀₂ is especially attractive for high powercommunications applications, where low divergence angles produceefficient collimation. The LP₀₂ mode in a fiber is significantly lessdivergent than the fundamental mode, and thus is well suited for thisapplication.

For the gain-block sub-assemblies described above to operate as anamplifier or laser, the dopants in the fiber are pumped with laser lightcorresponding to their absorption bands. This may be achieved by severaltechniques previously disclosed for pumping high power sources andamplifiers. FIG. 10 a shows a conventional pumping schematic used forpumping amplifiers for moderate as well as high power applications—theschematic most commonly used to pump erbium doped fiber amplifiers incommunications systems. Pump light from fiber-coupled laser-diodes ismultiplexed onto the input fiber of the gain block with a multiplexer.The multiplexer may act as both a wavelength—as well as apolarisation-selective element, thus enabling the prospect of addingseveral pump beams into the gain block. For even higher powerapplications, a fiber-tapered bundle may be used to introduce light frommany laser-diode pumps into the cladding of the gain fiber. Such devicesare well known. FIG. 10 b illustrates this schematic. In this case, theHOM fibers disclosed in this application will be coated with low-indexpolymer jackets so as to enable confining the pump light in the claddingof the fiber. FIG. 10(c) illustrates a side pumped laser with endmirrors. End pumping the laser, as suggested by FIGS. 10(a) and 10(b) isalso an option. Reflecting means other than mirrors, e.g. gratings, maybe used.

Methods for making optical fibers with the profiles shown here are wellknown and well developed. The core region generally consists of silicadoped with germanium at concentrations less than 10 wt % at the positionof maximum index, and graded with radius to provide the shape desired.The center core is typically has a radius of less than 10 microns. Theinner cladding region may be undoped, as in the case of the ring designshown in FIGS. 2 and 3, or lightly doped with germania as in the case ofthe truncated cladding design shown in FIGS. 4 and 5. In the ringdesign, the inner cladding extends between the core region and the ringto a radial distance of typically 20-50 microns. The ring is an updopedregion, usually doped with germania, with a radial width typically 1-5microns. In the truncated cladding design the ring is omitted. In bothdesigns the next region is a down-doped, typically fluorine-doped,trench region of considerable depth, i.e. at least 0.005 Δn from thedoping level of the inner cladding. The recommended radial width of thedeep trench is 5-15 microns. The index of refraction in the trenchregion is typically approximately constant as a function of radius, butis not required to be flat. The trench region generally consists ofSiO₂, doped with appropriate amounts of fluorine to achieve the desiredindex of refraction, and optionally germania to lower glass defectlevels.

As described in detail above, the optical fibers in the input, gain andoutput section of the device are designed to support specific guidedmodes. That characteristic, when specified herein, means that at least50% of the optical energy in the fiber is in the designated mode.

As used herein, the term ΔN refers to a percentage deviation from abaseline, the baseline being the refractive index of pure silica. Asevident from the description above, the optical fiber in the high-gainblock of the optical fiber device of the invention has a core, an innercladding, and a trench. Optionally, it has a ring between the innercladding and the trench to aid in controlling bend losses. Also, it mayhave an outer cladding outside the trench. The refractive index profileof the optical fiber can be expressed in terms of the radial position ofthese regions in microns.

Mentioned earlier is the use of a LOM input, for example a ring modefrom a high power laser. The input for the device of the invention maybe a LOM such as LP02, and the mode conversion to LP12, for example. Theoutput from the devices described above, i.e. the LP02 output, insteadof converting back to LP01, may be used as the input for a second stageof a two-stage amplifier,

Various other modifications of this invention will occur to thoseskilled in the art. In particular, the characteristics of modeseparation and selectivity may be advantageous even for moderatemodefield areas, between 350 and 1600 μm². This is because known designsfor fibers supporting HOMs with adequate mode separation and selectivityare restricted to A_(eff) of approximately 100 μm². In addition, it isexpected that alternate fiber designs can achieve grater than 2800 μm².All deviations from the specific teachings of this specification thatbasically rely on the principles and their equivalents through which theart has been advanced are properly considered within the scope of theinvention as described and claimed.

1. An optical device comprising: an input optical fiber supporting alower order mode (LOM), a first mode converter for converting the LOM toa high order mode (HOM), a gain section connected to the mode converter,the gain section comprising an optical fiber supporting the HOM, andhaving an Aeff of at least 350 microns².
 2. The optical device of claim1 wherein the gain section has an Aeff of at least 500 microns².
 3. Theoptical device of claim 1 wherein the optical fiber in the gain sectionis a few mode fiber.
 4. The optical device of claim 3 wherein theoptical fiber in the gain section supports LP02.
 5. The optical deviceof claim 4 wherein the input optical fiber supports LP01.
 6. The opticaldevice of claim 1 further including a second mode converter forconverting the HOM to a LOM.
 7. The optical device of claim 1 whereinthe optical fiber in the gain section comprises a core, an innercladding, and a trench.
 8. The optical device of claim 7 wherein theoptical fiber in the gain section additionally comprises a ring betweenthe inner cladding and the trench.
 9. The optical device of claim 7wherein the optical fiber in the gain section additionally comprises anouter cladding outside the trench.
 10. The optical device of claim 7wherein inner cladding of optical fiber in the gain section has a dopinglevel that is essentially flat and has a Δn of less than 0.002.
 11. Theoptical device of claim 7 wherein the inner cladding extends to a radiusof at least 20 microns.
 12. The optical device of claim 7 wherein theinner cladding and the core produce a mode field diameter of at least 40microns.
 13. The optical device of claim 3 wherein the trench has adepth at least 0.003 Δn from the inner cladding.
 14. The optical deviceof claim 13 wherein the trench has a width in the range 5-15 microns.15. The optical device of claim 3 wherein the mode converter comprises along period grating.
 16. The optical device of claim 1 in which the HOMfiber supports more than one mode with a difference in effective indexDn_(eff) between the dominant propagation mode and other modes isgreater than 8×10⁻⁵.
 17. The optical device of claim 1 in which the modeconverter functions as a wavelength selective filter.
 18. An opticalamplifier comprising the optical device of claim 1 and an optical pump.19. A laser comprising the optical device of claim 1, an optical pump,and optical reflectors at the input and output ends of the device. 20.The optical device of claim 1 wherein the fiber is doped with a rareearth selected from Er, Yb, Nd, Tm, Pr, Ho, Dy or combinations thereof.